1. Field of the Invention
This invention relates to a valve actuator that is electrically operated and used to selectively move a valve closure member in a gate valve between open and closed positions and thereby control flow through the valve. The actuator is used in emergency situations to quickly close the valve by tripping a release means that allows a coiled spring acting in combination with the residual pressure in the valve acting on the stem to rapidly move the valve closure member from a first, typically open, position to a second, typically closed, position.
Valve actuators that are hydraulically, pneumatically or electrically operated are well known and frequently used in the oil and gas industry. These valve actuators are required to be able to open the valve and hold the valve open against the full working pressure of the valve. This internal valve pressure may exert a substantial force against the actuator, trying to overcome the opening force generated by the actuator. Simultaneously, the actuator must be able to react in an emergency situation and close the valve in a matter of seconds.
This closing operation is typically accomplished by using a large diameter coil spring acting in combination with the residual pressure in the valve acting on the stem to provide the motive force to urge the valve to its closed or failsafe position in a few seconds. This spring must be sized to have sufficient force to close the valve at the valve""s rated working pressure. In the case where it is necessary to close the valve while the pressure in the valve is a relatively small percentage of the valve""s rated pressure, the spring is now somewhat oversized causing the valve to xe2x80x9cslamxe2x80x9d shut under a great accelerative force thereby causing increased wear on the valve actuator""s components. It is thus desirable to be able to control the rate at which the valve is closed and thereby minimize actuator component wear.
Previous attempts to control this closing force have focused on controlling the rate at which the actuator is closed. Various devices such as viscous rotary dampers, centrifugal clutches and centrifugal pumps with metering valves have been used. These devices suffer from various deficiencies including high breakout torque, large torque variance with temperature changes and leakage of the fluid used as the damping media. It is therefore desirable to have a clutching mechanism or similar apparatus that has a low breakout torque, is minimally affected by temperature changes and requires no fluids for operation.
2. Description of Related Art
U.S. Pat. No. 5,261,446 to Gerald S. Baker shows a self-contained emergency shutdown valve and actuator assembly with a hydraulic pilot pressure control system for determining when an emergency shutdown of the valve is required. Movement of the valve stem is done by use of a recirculating ball and nut shaft arrangement.
U.S. Pat. No. 6,152,167 to Gerald S. Baker discloses a valve actuator with an emergency shutdown feature that includes a wrap spring clutch to control movement of the actuator in one direction but not in the other unless acted upon by the tripping system.
U.S. patent application Ser. No. 09/915,796 by Johnny Newport and Todd Mosley, filed Jul. 26, 2001 and assigned to the same. assignee as the present application, shows a roller screw actuator for use with subsea chokes that uses a planetary roller screw to operate a choke.
The present invention comprises a valve actuator having a cylindrical housing with a bonnet secured to one end of the actuator housing. The bonnet has a stem bore therethrough and a stem positioned therein that is configured for attachment to a gate in a gate valve such that movement of the stem moves the gate of the gate valve between open and closed positions corresponding to operating and failsafe positions of the gate valve. A biasing module, positioned in the actuator housing, is connected to the stem and having a coil spring to urge the stem and valve gate to a closed, i.e., failsafe position. A transmission module including a low friction planetary roller screw is adjacent the biasing module and moves the stem and valve between the open and closed positions. Movement of the transmission module is opposed by the spring and the valve bore pressure. A power module drives the transmission module. An eddy current clutch is connected to the transmission module to control the movement of the stem at a preselected, linear rate as the stem is moved by the spring to the failsafe position. A pair of wrap spring clutches are used to restrain and control movement of the planetary roller screw. A control module is provided to allow selective release of the wrap spring clutch and thereby allow the spring to move the stem and valve gate to their failsafe position.
A principal object of the present invention is to provide an electrically operated valve and actuator assembly with a clutch apparatus having a substantially linear torque versus speed relationship.
Another object of the present invention is to provide an electrically operated valve and actuator assembly with a clutch apparatus with a low breakout torque.
A further object of the present invention is to provide an electrically operated valve and actuator assembly with a clutch apparatus that is minimally affected by temperature changes.
A final object of the present invention is to provide an electrically operated valve and actuator assembly with a clutch apparatus that requires no fluids for operation.
These with other objects and advantages of the present invention are pointed out with specificness in the claims annexed hereto and form a part of this disclosure. A full and complete understanding of the invention may be had by reference to the accompanying drawings and description of the preferred embodiments.